Interest in wireless (i.e., inductively coupled) battery charging for consumer electronic devices has been increasing. Wireless power transfer circuits (e.g., wireless battery chargers) have often been designed with a high DC voltage input, e.g., (20V to 200V or more). However, the output voltage level of these wireless power transfer circuits may be much lower, although with a relatively high output current. As a result, conduction losses on the receiver side of the wireless power transfer circuit become more and more important as output power levels increases. (Conduction losses are proportional to the square of the current.)
Conventional wireless power transfer circuits often use a full bridge rectifier on the receiver side of the circuit. At low power levels, a full bridge rectifier constructed using conventional diodes may be adequate. As power levels increase, other solutions may be appropriate, such as full bridge rectifiers constructed from Schottky diodes or a full bridge synchronous rectifier constructed using metal-oxide field effect transistors (MOSFETs). Both of these arrangements can reduce the voltage drop across the rectifier components, leading to an associated reduction in conduction losses. In some cases, the synchronous rectifier control circuitry and the MOSFETs may be available as a single integrated package.
However, as output power levels increase even further, discrete external MOSFETs may required because of thermal issues associated with integrating high power switches with the associated controller circuitry. Additionally, high voltage driver circuitry may be required because the gate driving voltage of the MOSFETs must be higher than the output voltage level. This high voltage driver circuitry can include charge pumps, boot strapping, or other means of deriving the increased voltage necessary to drive the high side switching devices. Additionally, controlling the short dead time required between high side and low side turn on (to prevent cross conduction) can be more difficult with external devices (as opposed to devices integrated in the controller silicon itself.) As a result, the receiver circuitry of higher power wireless power transfer systems becomes more complex and more costly.
Thus, a more efficient, simplified wireless power transfer receiver circuit would be a desirable addition to the art.